-If u have windows, use winscp.
-If you have mac, use cyberduck.
-On your phone, make sure open ssh is installed.
-Use one of the above mentioned programs to access your phone's system files
-for iphone 4, the image files that are used are going to end with @2x.png. Most app icons are going to be called icon@2x.png.
-for non iphone 4, its just icon.png etc
-just replace the icon with the custom one.
-whenever you change files, you always have to respring for changes to take effect
CAUTION: WHEN CHANGING SYSTEM FILES, ALWAYS MAKE BACKUPS BEFORE CHANGING.